ProtocolComputers can't just throw data at each other any old way. Because so many different types of computers and operating systems connect via modems or other connections, they have to follow communications rules called protocols. The Internet is a very heterogeneous collection of networked computers and is full of different protocols, including TCP/IP, UDP, PPP, SLIP, and FTP.

Data packetAlthough your computer and modem can send data one character at a time, when you're surfing the Internet, downloading files, or sending email, it's more efficient to send information in larger blocks called data packets. Modems generally send packets of around 64 characters along with some extras for error checking. When downloading files using a protocol like Xmodem, however, the packets are larger. And when using Internet protocols such as TCP/IP, the packets are larger still--around 1,500 characters.

ClientThe customer side of a client/server setup. To confuse matters, when you log on to a server, the word client can refer to you, to your computer, or to the software running on your computer. For example, to download something from an ftp site, you use ftp client software.

ServerThe business end of a client/server setup, a server is usually a computer that provides the information, files, Web pages, and other services to the client that logs on to it. (The word server is also used to describe the software and operating system designed to run server hardware.) The client/server setup is analogous to a restaurant with waiters and customers. Some Internet servers take this analogy to extremes and become inattentive, or even refuse to serve you.

Asynchronous communicationThis term describes how your computer uses a modem to connect with other computers. Back in the days of teletypes and dumb terminals, computers sent data synchronously--they operated using a shared timer that marked the transmission of each character. This didn't work very efficiently for large blocks of data over phone lines, however. So modern modems use asynchronous rules: instead of synching up to a time signal to mark a character, transmitting computers use a start bit, a stop bit, and an optional error-checking parity bit to indicate to receiving computers the boundary of each character. (The term is a bit of a misnomer, though, since all modems synch up with one another before they transmit data.)

ParityThis is an obsolete method of detecting communication errors. These days, communication ports are almost always set to No Parity, and the modem's internal error detection and correction are used to provide reliable communication.

Start bitAncient modems alternated between two tones. One tone represented 0 and the other represented 1, which covered the entire binary language. Silence was not an option: to a modem, silence meant the phone call had been cut off. So when a modem had nothing to say, it continuously transmitted the 1 tone to tell the other modem that it was in an idle state. When there was more data to send, the modem would send a single 0 bit to say there was real data on the way. The next eight bits the receiving modem heard were the real data. Then the line went back to idle for at least one more bit. The 0 just before the data is called the start bit. The 1 at the end is the stop bit.

Stop bitThe opposite of the start bit.

Parity bitBecause old modems transmitted data one character at a time, each character had to have its own individual error check. The usual rule was to add an extra bit (the parity bit) at the end of each character before the stop bit. This bit would be set to 0 or 1 based on the value of the previous data bits. These days, modems gather data into packets and send a larger 2- or 4-byte error-check value to validate data in the entire packet. So a parity bit is no longer necessary.

CRCCyclical Redundancy Check CRC is a mathematical technique used to check for errors when sending data by modem. Because some phone lines are notoriously crackly and can cause breaks in transmission, this is a crucial step. If the CRC fails to add up, the receiving end of a data transmission sends a NAK (negative acknowledgement or "say that again") signal until it does add up. CRCs are also used in tape backups and other streaming communications.

StreamingData is streaming when it's moving quickly from one chunk of hardware to another and doesn't have to be all in one place for the destination device to do something with it. When your hard disk's data is being written to a tape backup device, it's streaming. When you're watching a QuickTime movie on the Internet, it's not streaming, because the movie must be fully downloaded before you can play it.

ACKAcknowledgement When a modem receives a data packet, it sends a signal back to the sending modem. If all the data is present and correct, it sends an ACK (acknowledgement) signal, which acts as a request for the next data packet. If the modem didn't get all the data, it sends back a negative acknowledgement, or NAK

NAKNegative Acknowledgement When a modem receives a data packet, it sends back a signal to the sending modem. If everything is in order, the signal is an ACK, or acknowledgement. If some of the data is missing or corrupt, the modem sends back a negative acknowledgement, or NAK, which acts as a request to resend the data.BandwidthThe amount of data that can be pushed through a link in unit time. Usually measured in bits or bytes per second.

LatencyThe amount of time that a message spends in a network going from point A to point B.

JitterThe effect seen when latency is not a constant. That is, if messages experience a different latencies between two points in a network.

RPCRemote Procedure Call. RPC is a method of making network access to resource transparent to the application programmer by supplying a "stub" routine that is called in the same way as a regular procedure call. The stub actually performs the call across the network to another computer.

MarshallingThe process of taking arbitrary data (characters, integers, structures) and packing them up for transmission across a network.

MBONEA virtual network that is a Multicast backbone. It is still a research prototype, but it extends through most of the core of the Internet (including North America, Europe, and Australia). It uses IP Multicasting which is defined in RFC-1112. There are frequent broadcasts of multimedia programs (audio and low bandwidth video) over the MBONE. Though the MBONE is used for mutlicasting, the long haul parts of the MBONE use point-to-point connections through unicast tunnels to connect the various multicast networks worldwide.

RFCRequest For Comment. RFCs are documents that define the protocols used in the IP Internet. Some are only suggestions, some are even jokes, and others are published standards. Several sites in the Internet store RFCs and make them available for anonymous ftp.

Quality of service (QOS) In the field of telephony, telephony quality of service QoS was defined in the ITU standard X.902 as "A set of quality requirements on the collective behavior of one or more objects."

In the fields of packet-switched networks and computer networking, the traffic engineering term Quality of Service (QoS) refers to control mechanisms that can provide different priority to different users or data flows, or guarantee a certain level of performance to a data flow in accordance with requests from the application program. Quality of Service guarantees are important if the network capacity is limited, especially for real-time streaming multimedia applications, for example voice over IP and IP-TV, since these often require fixed bit rate and may be delay sensitive.

VLANA VLAN is a switched network that is logically segmented by functions, project teams, or applications without regard to the physical location of users. For example, several end stations might be grouped as a department, such as engineering or accounting. When the end stations are physically located close to one another, you can group them into a LAN segment.

VPN and Network Tunneling Now, many companies are creating their own VPN (virtual private network) to accommodate the needs of remote employees and distant offices. Basically, a VPN is a private network that uses a public network (usually the Internet) to connect remote sites or users together.

RAIDA Redundant Array of Independent Drives (or Disks), also known as Redundant Array of Inexpensive Drives (or Disks), (RAID) is an umbrella term for data storage schemes that divide and/or replicate data among multiple hard drives. RAID can be designed to provide increased data reliability and/or increased I/O performance.

OSI (The Open Systems Interconnection Model) The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnection initiative. It is also called the OSI seven layer model.